Method for treating electrophotographic recording materials with protective overcoatings

ABSTRACT

IN A RECORDING ELEMENT FOR ELECTROPHOTOGRAPHIC MARKING HAVING ON AN ELECTRO-CONDUCTIVE PLATE SUCH AS A METALLIC PLATE, A PHOTOCONDUCTIVE LAYER COMPOSED OF A COMPARATIVELY LARGE QUANTITY OF ZNO AND AN EPOXY-ESTER RESIN BINDER, THE DETERIORATION OF THE PROPERTIES OF THE LAYER, SUCH AS ADHESIVITY TO A PRIMER COATING CAUSED BY THE HIGH CONTENT OF ZNO AND THE OCCURRENCE OF FAULTS CAUSED BY THE WEAK ADHESIVITY OF THE LAYER ARE PREVENED BY APPLYING A THIN EPOXY-ESTER RESIN COATING TO THE LAYER, AFTER FORMING A VISIBLE IMAGE ELECTROPHOTOGRAPHICALLY.

United States Patent 3,552,956 METHOD FOR TREATING ELECTROPHOTO- GRAPHIC RECORDING MATERIALS WITH PROTECTIVE OVERCOATINGS Shigeru Sadamatsu, Odawara-shi, and Saturn Honjo, Kita- Adachignn, Saitama, Japan, assignors to Fuji Shashin Film Kabushiki Kaisha, Kanagawa, Japan No Drawing. Filed Jan. 25, 1967, Ser. No. 611,563 Claims priority, application Japan, Jan. 25, 1966, 41/4,244 Int. Cl. G03g 13/22 U.S. Cl. 96-1 6 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a new method for treating a light sensitive layer of an electrophotographic recording material to improve the adhesion of that layer with an overcoating formed thereon.

(2) Description of the prior art It is necessary in the aircraft industries or ship building industries to cut or process metal plates of relatively large size according to design drawings. Conventionally, drawings of plan have been reproduced in actual size by hand marking on the materials to be processed. They are then processed according to the marked drawings. Such a hand marking procedure, however, took time and and prodigious labor, and moreover, was inevitably accompanied with errors. In some cases, a three-dimensional model of actual size had to be prepared.

More recently an automatic flame cutter was introduced, in which a light beam transmitting a photographic negative of a drawing of reduced size regulates the movement of a burner directed toward the surface of a metal plate to be cut. This procedure requires the additional process of marking working directions or classification numbers for cut pieces.

The inventors have established a method of electrophotographic marking for use in processing conductive materials as is described in Japanese Pat. 39/ 6,079, wherein an electrophotographic layer is provided on a conductive material to be processed on which drawings of plan as well as working directions and other information are reproduced electrophotographically. Such a marking process is now being utilized in the ship building industries.

An electrophotographic layer for use in this marking process which comprises photoconductive pulverized materials and an insulating binder must satisfy many requirements. First of all the electrophotographic layer must he possessed of excellent electrophotographic properties. Next, since the marking has to be done at an early stage of processing of the materials, the light sensitive layer must also work as a satisfactory primer coating on which an overcoating or surface coating is to be applied. This 3,552,956 Patented Jan. 5, 1971 requirement is eliminated when the marking layer is removed or dissolved off.

Suitable compositions of ZnO in a binder layer were devised by selecting the insulating binder materials, and the ratio of the binder to the photoconductive ZnO to meet these requirements. Recently, however, a further requirement has arisen to shorten or minimize the time required for the electrophotographic marking process. It has become impossible to meet this added demand solely by improving the composition of the layer.

The inventors have found a Way to solve this difiicult problem as a result of an extensive investigation, which makes it possible to impart improved adhesion of an electrophotographic layer to many kinds of overcoating applied thereon by subjecting it to a treatment, after the marking procedure has been completed.

SUMMARY OF THE INVENTION Accordingly, the present invention provides an improved electrophotographic marking process which is free from the shortcomings of conventional methods, and is characterized by forming on an electrically conductive or slightly conductive substrate an electrophotographic layer comprising about parts by weight of ZnO and about 40 parts by weight or less of binding material which substantially comprises an epoxy-ester resin, performing the electrophotographic procedure therewith, thus forming a visible image, then applying uniformly on the electrophotographic layer an organic solution containing filmforming material dissolved in it, the film-forming material substantially comprising an epoxy-ester resin, so as to give a thin resin film with a dried thickness of 0.5 to 10 microns.

The last treatment by epoxy-ester resin solution insures an improved adhesion between the photoconductive layer and a variety of coatings subsequently formed thereon. Therefore, the principal object of the present invention is to provide a new method of marking and processing conductive materials, such as steel or many light metals used for ship building or airplane or plant construction. Another o'bject of the present invention is to provide a new method of toner fixation.

DESCRIPTION OF PREFERRED EMBODIMENTS An electrophotographic layer or coating mixture to form such a layer for use in electrophotographic mar-king of a structural material of a large size which is fed to a marking installation which marks at high speed and is continuously operated, must satisfy the following requirements:

(1) The coating mixture must be stable during prolonged storage, without causing precipitation of pigment therein or deteriorating for electrophotographic properties. Even if the precipitation of pigment occurs, it should be easily redispersed.

(2) The coating mixture must be adaptable to an automatic coating apparatus, and have good working properties for coating.

(3) The mixture must be so constituted that it provides a layer which dries sufliciently fast, showing good electrophotographic properties (i.e. high potential acceptance) as quickly as possible after coating. This requirement is very important to shorten the time required for electrophotographic marking, since ordinarily the drying step is rate-determining.

(4) The coated layer must be insensitive to a relatively strong pre-exposure to light before charging, i.e. it must show a rapid recovery from light fatigue.

(5) The electrophotographic properties of the layer should be rather insensitive to changes in relative humidity or temperature under which the layer is formed or used.

In addition to the above requirements with respect to electrophotographic properties, the layer should satisfy the following requirements as a primer coating on a metal substrate:

i (1) It must firmly adhere to a variety of conventional primer coatings, such as, for example, a wash primer or anti-rust primer, as well as to a bare metal surface.

(2) It must also accept a variety of overcoatings provided thereon, forming firm adhesion.

(3) It is desirable that the marking layer itself have rust preventive properties.

(4) It should not be deeply colored or generate harmful decomposition products when it is subjected to flame cutting or welding.

(5) The presence of the layer must not have any adverse effect on the mechanical strength of welded portions of the structure.

The inventors have discovered as a result of an extensive study that a binder composition comprising mainly an epoxy-ester of fatty acids of vegetable oils (epoxyester resin) can satisfy almost all of the above requirements when used as a binder for pulverized ZnO.

More specifically in accordance with the invention it has been found that:

(1) Epoxy-ester resin can stably disperse ZnO particles. The addition of a dispersing agent, such as copper Stearate, can further improve this property. A precipitated cake, once formed, proved to be easily redispersible.

(2) The coating mixture comprising ZnO and epoxyester has excellent working properties for automatic coating apparatus as well as hand spraying; since this resin needs no hardening reagents or catalysts, the viscosity change in a storage tank is very small. It has little tendency to form a blushed coating under high humidity conditions.

(3) The drying speed of ZnO/epoxy-ester coating is remarkably influenced by the mixing ratio of ZnO to the binder; i.e. the higher the content of ZnO in the coating, the greater the drying speed of the resulting coating. When the mixing ratio of ZnO to binder exceeds about 100130 to 25, a very rapid drying property is imparted.

An electrophotographic layer comprising ZnO and epoxy-ester resins are already described in United States patent specifications No. 3,152,894 and No. 3,152,895, which also state the rapid drying property of such coatings. However, the meaning of rapid drying used in these patents is that a coating very quickly becomes nontacky at its surface along with vaporization of a solvent, after being applied on a substrate as a thin film of a coating mixture.

The present rapid drying property refers to the short time interval required for an electrophotographic layer to show such a high insulating property that it can serve as a hotoconductive insulating layer. This property has proved to be more closely related to the ratio of ZnO to binder than with the type of binder.

(4) A ZnO coating utilizing epoxy-ester resin as a binder is in general insensitive to pre-exposure when the resin, which hardens gradually under the influence of oxygen in air, is not completely hardened, provided that the purities of the ingredients of the layer are sufliciently high. Such a layer proved to have a potential acceptance, immediately after exposure to sunlight, of about several times ten thousand luxes for a few minutes, as high as it does when it is dark adapted.

(5) When one constitutes the binder composition solely from epoxy-ester resin, the resulting layer shows inferior properties under highly humid conditions; this defect, however, can be eliminated by adding a polysiloxane resin which is compatible with the epoxy-ester used. Very costly polysiloxane resin need not be used if the whole procedure of electrophotographic marking is carried out in a controlled low humidity atmosphere.

Sensitivity to humidity is remarkably decreased when polysiloxane resin is introduced in an amount of from about 10 to 50% of the whole binder. Since polysiloxane resins generally cause deterioration of adhesive properties of the layer with other coatings, the amount should be kept as low as possible.

(6) The layer of the present composition shows firm adhesion with conventional primer coatings.

(7) The layer of the present invention failed to meet this requirement especially when the content of ZnO is high. To meet this requirement the introduction of an after-treatment is necessary.

(8) This requirement is in general inconsistent with the third, high drying speed requirement. When the content of ZnO is high, and the thickness of the layer is about 10 to 15 microns on a dry basis, the rust preventive property of the layer is far inferior to that of a conventional primer. Particularly when a steel plate coated with a ZnO/epoxy-ester marking layer is subjected to a rather severe rust-preventive test, such as salt water spraying test, it does not give a satisfactory result at all. Accordingly it is recommended that such a layer be provided on a wash primer layer. If a long period of drying is permitted, a greater thickness of the layer may be used.

(9) A layer of the present composition is rendered slightly colored by a high temperature treatment, but the degree of discoloration is permissible for successive processing. It does not generate any harmful decomposition product by such treatments.

(10) The presence of the present layer proved not to exert any adverse effect on the mechanical properties of welded portions of a structure provided that the thickness of the layer does not exceed 40 to 50 microns.

As will be seen from the above descriptions, a ZnO/ epoxy-ester layer satisfies all the requirements except the adhesion property with overcoatings (7) and the rust preventive property (8).

From the fact that the higher content of ZnO in the layer resulted in a poor adhesion with many overcoatings which firmly adhere to layers consisting of epoxyester resin only, it may be concluded that the adhesive property of a ZnO/epoxy-ester marking layer is mainly determined by the presence of ZnO. From this point of view, it has been tried to apply a very thin (about 2 or 3 microns) layer of a variety of film-forming resinous materials on the ZnO/epoxy-ester layer to form an intermediate layer between the marking layer and the overcoating.

An extensive investigation showed that an intermediate layer comprising epoxy-ester resin can remarkably improve the adhesion of these two layers.

EXAMPLES tent in varnish 50%) 40 Polysiloxane resin varnish compatible with the epoxy-ester (nonvolatile content in varnish '60%) 8.3 Toluene 20 Copper Stearate 0.1

Before air spraying, these ingredients were thoroughly mixed and diluted with toluene to give a suitable consistency for coating.

This thickness of the marking layer was about 12 to 15 microns after drying. After the volatile solvent was evaporated, the coated plate was subjected to a negative corona discharge at darkness, then exposed to light through a positive transparency. Cascade development was carried out and a positive toner image was obtained.

Then an organic solvent solution of the same epoxyester used in the photoconductive layer was sprayed on the toner image-bearing layer, forming an intermediate layer. At the same time the fixation of the toner image was also accomplished.

For the purpose of comparison other film-forming materials were applied as the intermediate layer (see Table I) to other plates processed similarly. As the standard or blank, one plate was sprayed with trichloroethylene to fix the toner image.

The sample plates thus prepared were held at room temperature for one week, then a variety of overcoatings were applied on the sample plates.

When two days elapsed after final coating, a salt spray test was carried out on these samples using 3% NaCl solution. The sample plates subjected to this test were dried then subjected to cross-cutting with a 3 mm. interval.

The final results are listed in Table I wherein A means that the plate did not show any peeling off, B that the plate showed some defect in cross-cut test, and C that the plate showed peeling off between coated layers.

advantageous property to an electrophotographic layer containing a greater amount of ZnO. Since such a layer is very porous and liable to generate fumes which contain finely-divided ZnO and are harmful to human health, when subjected to gasflame cutting or welding. This defect occurs by accident when the coating condition of the electrophotographic layer is not suitably controlled. Such a problem can be removed by providing an intermediate layer thereon.

What we claim is:

1. An electrophotographic method for marking a pattern on a light sensitive layer overlying a metallic substrate which comprises electrophotographically forming a visible image on an electrophotographic recording material comprising a metallic support bearing thereon a light-sensitive layer containing about 100 parts by weight of zinc oxide and less than about parts by weight of a binder, said binder substantially comprising an epoxyester resin, uniformly applying to said layer an intermediate coating of an organic solvent solution of an epoxy-ester resin, drying said coating to provide a thin Table I Intermediate layer Drying oil Styrenated Polyvinyl overcoating None Epoxy-ester mod. alkyd alkyd butyral Evabond (Chugoku Tory C A C C C Rust preventive paint for inboard (Nippon OllS & Fats Co.). B A A A B Zep (Nippon Oils & Fats Co.) B A B B 13 Hold paint (Nippon Oils & Fats Co.) C A A A B Wash coat (Hempel Co.) O A B C B Hempalin primer (Hempel C0.) C A A C A Chrorno primer (Yoton C0.) B A B B B Rustop Snow (Kawakami Toryo Co.) A A A A A Balasco #9400 (Ni pon Oils & Fats Co.) A A A A A Tar-Epoxy RT-50 (Hirata Chemical)..- A A A A A Shinseil (Toa Paint 00.). C A C C Epoxy-ester resins used for the present invention are obtained by reacting epoxide resins and fatty acids (saturated or unsaturated) contained in many kinds of fatty oils such as dehydrated castor oil, soya oil, linseed oil, oiticica oil, tung oil, rosin, hydrated rosin, or coconut oil; they may be modified by polymerizable vinyl monomers such as styrene or acryl esters. When the content of fatty acids is high, the resulting ester can be cured by the pres ence of oxygen. For the present purpose, however, short oil length (preferably less than 60%) epoxy-ester is recommended, since an oxygen cured coating sometimes causes lifting under the influence of strong solvents, such as Cellosolves, methyl ethyl ketone, or butanol used for overcoating.

A further test proved that intermediate layers containing not less than 60% epoxy-ester can improve adhesion between ZnO/epoxy-ester and overcoatings.

The theoretical interpretation of the effect of the intermediate layer is unclear at present; but the following observations may partially explain the eifect. Photoconductive ZnO present in a photoconductive insulating layer has a strong tendency to decompose photocatalytically its binder material. This function is promoted by the presence of oxygen. It is found that this photocatalytic decomposition is greatly retarded as the ratio of ZnO to binder in the layer decreases. The treatment of the present invention may perhaps be considered to decrease this ratio at the surface of the layer. It should be taken into consideration that the intermediate layer is applied before the binder in the electrophotographic layer has been hardened. It is also important that the intermediate layer has a strong resistance to alkaline reagents such as ZnO. Regardless of the theory, the epoxy-ester intermediate layer proved to enhance adhesion between the ZnO/epoxy-ester layer and overcoating layers.

The intermediate layer may be prepared in another procedure than toner fixation.

The thin intermediate layer sometimes imparts another resin film having a dried thickness of from about 0.5 microns to about 10 microns, and further applying a protective overcoating to said thin resin film.

2. The method as claimed in claim 1 wherein the electrophotographic formation of the visible image is conducted before said epoxy-ester resin in the light-sensitive layer is completely cured.

3. The method as claimed in claim 1 wherein the electrophotographic formation of the visible image and the application of said organicsolvent solution are conducted before the epoxy-ester resin in the light-sensitive layer is completely cured.

4. The method as claimed in claim 1 wherein said binder has incorporated in it from about 10% to by weight of the binder of a polysiloxane resin compatible with said epoxy-ester resin.

5. The method as described in claim 1 wherein said epoxy-ester resin in the binder is prepared from a dehydrated castor oil fatty acid, the oil length of which is less than 6. The method as described in claim 1 wherein said epoxy-ester resin in said organic solvent solution is prepared from a dehydrated castor oil fatty acid, the oil length of which is less than 60%.

References Cited UNITED STATES PATENTS 2,990,280 6/1961 Giaimo 96l 3,152,894 10/1964 Tinker et al. 961 3,152,895 10/1964 Tinker et a1. 96-1 3,206,600 9/ 1965 Gold 250 DONALD LEVY, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R. 961.8; ll7--17.5 1 

